Switching converters are widely used in various electronic products for providing stable direct-current (DC) voltages for load circuits such as central processing unit (CPU). FIG. 1 shows a conventional synchronous buck converter 10, in which a control circuit 12 produces two control signals UG and LG to switch an upper switch SW1 and a lower switch SW2 in a power stage 14 so as to convert an input voltage Vin to an output voltage Vout, and by detecting the output voltage Vout a feedback signal FB is fed back to the control circuit 12 for regulating the output voltage Vout. FIG. 1 also shows a power stage 16 of a synchronous boost converter and a power stage 18 of a synchronous inverting converter. If the switches SW2 in the power stages 14-18 are replaced by diodes, these converters will be asynchronous ones. It is well know by those skilled in the art that there had been many control methods for these converters.
FIG. 2 shows a conventional control method for a constant on-time PWM switching converter. In steady state, each switching cycle of the control signal UG has a constant on-time Ton followed by an off-time Toff. The power switch controlled by the control signal UG is turned on during the on-time Ton, and is turned off during the off-time Toff. The process of producing the on-time is described as below. The control circuit 12 compares the feedback signal FB with a reference signal Vref to produce a pulse modulation signal PM. When the feedback signal FB decreases lower than the reference signal Vref with the decreasing output voltage Vout, the pulse modulation signal PM will change to high level, so as to trigger the on-time Ton. During the on-time Ton, the feedback signal FB gradually rises up with the gradually rising output voltage Vout, and when it becomes higher than the reference signal Vref again, the pulse modulation signal PM changes back to low level, but the on-time Ton still keeps, hence the output voltage Vout keeps rising up until the on-time Ton terminates when it reaches the default length, and then gradually decreases again thereafter. When the on-time Ton terminates, the control signal UG falls down to low level, so triggering a minimum off-time Toff_min. When the minimum off-time Toff_min terminates, the feedback signal FB is still higher than the reference signal Vref, so the control signal UG is also still at low level. The control signal UG keeps at low level until the next time the feedback signal FB decreases lower than the reference signal Vref, which changes the pulse modulation signal PM to high level and so triggers the on-time Ton again.
When the load changes from light to heavy, the load current Io increases rapidly, causing the output voltage Vout and thereby the feedback signal FB dropping down rapidly. The pulse modulation signal PM will keep at high level until the feedback signal FB increases higher than the reference signal Vref again. It causes that in the control signal UG, the constant on-time Ton and the minimum off-time Toff_min are alternatively triggered instantly, without more time therebetween for the off-time, which increases the switching frequency during the transient state, resulting in more switching loss of the switches SW1 and SW2, and so decreasing the efficiency of the converter. Moreover, during the transient state, because the on-time Ton for the output voltage Vout to increase is constant, it needs more switching cycles or longer time interval for the output voltage Vout to recover to the previous level, and thus the response to the load transient will be slow.
Therefore, it is desired a control circuit and method for a constant on-time PWM switching converter with decreased switching frequency in the transient state and reduced recovery time for the output voltage.